1. Field of the Invention
This invention relates to methods of forming composite magnetic compacts consisting essentially of a powder of superplastic Zn-22Al alloy, a magnetic powder and a powder of plastic, and more particularly to new composite magnetic compacts having good conductivity and excellent mechanical and magnetic properties and their forming methods.
2. Description of the Prior Art
Magnetic materials can generally be classified into hard magnetic materials having a coercive force of approximately 100 Oe or above and soft magnetic materials having a lower coercive force. The former includes ferrite magnet, sintered alnico magnet and rare-metal cobalt magnet which are known as magnet materials. They have wide applications in various kinds of electric appliances, measuring instruments, communications equipment, audio equipment, attracting magnets, toys and the like. Soft magnetic materials are used for transformers, magnetic heads, dampers, electromagnetic-wave absorbers and so on.
In recent years, general choices of market has shifted from heavy, thick, long and large products to lighter, thinner, shorter and smaller ones. The same applies to products made of magnetic materials. Demand for products smaller in size, higher in performance and more intricate in profile is increasing. Such demand can be satisfied, for example, by injection-moulding that is applied to such hard magnetic materials as plastic and rubber magnets.
In injection-moulding, a powder of hard magnetic material, such as a powder of ferrite, is mixed and stirred with molten thermoplastic or thermosetting plastic. The mixture is extruded into a mold placed in a magnetic field. After the profile of the mold has been transferred, the compact is cooled to room temperature, and then magnetized in a strong magnetic field. This method permits producing in a single process a compact of intricate profile that is very close to the desired final product in shape and dimension.
But injection-moulding and resins formed thereby have some drawbacks: (1) Serving as insulators, such resins do not prevent electromagnetic interference that has been at issue recently; (2) Losing plasticity on being formed, such resins are practically un-reformable; (3) Such resins must be kept above their melting point while they are being formed, as a consequence of which heating energy constitutes a large percentage of the total energy consumed in forming; (4) Compacts of such resins do not have high toughness; (5) Mold design must allow for the shrinkage of compacts and, therefore, require highly sophisticated skill; and (6) Compacts are likely to have flashes produced thereon as a result of injection-moulding.
Recently, more and more OA (office automation) machines and the like have come to be made of plastic, creating a serious social problem of electromagnetic interference. Therefore, development of effective electromagnetic shielding means has been expected. Another social problem coming up lately is vibration obstacle that also calls for the development of appropriate vibration absorbers and dampers.